North Korea attempted to fire a missile from its east coast early on Tuesday morning but the launch appears to have failed, South Korean military officials told Reuters.
The launch attempt took place at around 5:20 a.m. Seoul time (2020 GMT), said the officials, who asked not to be identified, without elaborating.
Japan put its military alert on Monday for a possible North Korean ballistic missile launch, state broadcaster NHK reported.
Tension in Northeast Asia has been high since North Korea conducted its fourth nuclear test in January and followed that with a satellite launch and test launches of various missiles.
South Korea's Yonhap News Agency said it appeared North Korea had attempted to launch an intermediate-range Musudan missile. North Korea attempted three test launches of the Musudan in April, all of which failed, U.S. and South Korean officials have said.
North Korea has never had a successful launch of the Musudan, which theoretically has the range to reach any part of Japan and the U.S. territory of Guam.
The attempted launch took place near the east coast city of Wonson, one of the South Korean officials said, the same area where previous Musudan tests had taken place.
The flurry of weapons technology tests this year had come in the run-up to the first congress in 36 years of North Korea's ruling Worker's Party early this month. Tuesday's attempted launch appears to have been its first missile test since then.